AFAIK you can't create timer in multiboard like you can easily create a single timer in a timer window, however it's not that complicated to imitate one - simply fill one column with the time left and for example every 0.1 second update the multiboard.
You can either have 1 loop trigger per timer - it's easy to set up; or have 1 loop trigger for all timers - this requires a bit of logic to put behind it, thus complicated things a tiny bit, but it's far more effective.